1. Field
Microelectronic circuit design and packaging.
2. Background
Some microelectronic devices may be placed on opposite sides of a motherboard instead of being placed side by side. For example, two individually packaged memory chips are sometimes mounted back to back on a motherboard. One of the chips is mounted facing up on top of the board, and the other chip is mounted facing down on the bottom of the board. The stacked configuration is area efficient in that it increases device density on the motherboard and allows manufacturers of computers, cell phones, portable music players, and other microelectronics to create smaller devices. The stacked configuration may simplify signal routing because the devices may share the same signal conducting lines on the motherboard. To improve high speed signaling characteristics, it is desirable to route conducting signals directly through the motherboard. However, this requires devices with different package pin-outs or other accommodation as the bottom device pins will be a mirror image of the top device pins.
A stacked device generally has a “normal” configuration and a “rotated” configuration. For example, when two Dynamic Random Access Memory (DRAM) devices are stacked, one of the DRAMs is rotated such that outputs from one device align with inputs to the other device. Sometimes, devices may not align correctly when one of them is rotated. The mismatch can be resolved by duplicating pin functions such that pin layout is the same in normal and rotated orientations. However, pin duplication creates a large number of redundant pins and is therefore highly undesirable for cost sensitive devices.
Another technique is to create input/output (I/O) designs that are capable of operating in two orientations. A conventional approach is to use one or more dedicated pins to determine orientation and then assign pin functions accordingly. For example, if a dedicated pin is pulled high, the device senses one orientation; if the pin is low, the device assumes the other orientation. One disadvantage of this approach is that the design requires at least one additional pin, or more likely two duplicate pins for symmetry. A further disadvantage is that the dedicated pins cannot be used for active signaling or other useful purposes after the orientation has been established.